1. Field of the Disclosure
The present disclosure relates to obtaining measurements of properties of a formation surrounding a wellbore using a propagation resistivity tool conveyed on a measurement-while-drilling device. In particular, the present disclosure discusses obtaining additional induction measurements at an increased depth of investigation using a propagation resistivity tool.
2. Description of the Related Art
This disclosure relates to the measurement of geophysical parameters of earth formations penetrated by a borehole and more particularly to propagation resistivity measurements at multiple transmitter frequencies and multiple transmitter receiver spacing. Measurements are typically made using an array of four transmitters and two receivers. Elements of the transmitter receiver array are longitudinally and symmetrically spaced along an elongated borehole instrument. Each transmitter induces an alternating voltage into the borehole and the earth formation in the vicinity of the borehole. The amplitudes and phase shifts of the signals produced by these induced alternating electromagnetic fields are measured by the receivers. These signals are affected by numerous formation, near borehole and borehole parameters. The measurements are combined to yield resistivity of the formation, parameters relating to the invasion of drilling fluids into the formation in the near borehole region, and physical characteristics of the borehole itself. The disclosure is directed toward, but not limited to, measurement while drilling (MWD) applications.
A complete description of an exemplary multiple propagation resistivity (MPR) tool is given, for example, in U.S. Pat. No. 5,869,968 to Brooks et al., having the same assignee as the present disclosure and the contents of which are incorporated herein by reference. U.S. Pat. No. 5,892,361 to Meyer Jr. et al, having the same assignee as the present disclosure and the contents of which are incorporated herein by reference, discloses a propagation resistivity measurement-while-drilling device used to measure borehole environmental parameters along with electromagnetic properties of the formation. Multiple transmitter-receiver pairs operating at one or more transmitter frequencies are used to obtain a set of amplitude and phase measurements. A model of the response of the borehole instrument in varying formation and borehole conditions is also utilized. Formation and borehole parameters of interest are selected by the analyst. The measured amplitude and phase data set is then combined with the model of the tool response to obtain the selected parameters of interest.
In one method of operation disclosed in Brooks, each transmitter is sequentially activated while the other transmitter is decoupled to eliminate mutual coupling, and the recorded signals processed to take advantage of reciprocity relations. In another method of operation, both transmitters are operated simultaneously with one relative polarity and then with another relative polarity, to eliminate the effects of mutual coupling and to take advantage of reciprocity relations. The process of compensation and use of the reciprocity relations reduces the redundancy that is inherent in the data. The present disclosure uses the redundancy to advantage and, additionally, is able to provide greater depth of investigation into the earth formation.